objcopy: Allow making symbol global and weak on same invocation

Previously objcopy had to be run twice in order to make a local symbol
weak, first once to globalize it, and once again to mark it as weak.

	* objcopy.c (filter_symbols): Weaken symbols after making
	local/global changes.
	* testsuite/binutils-all/symbols-5.d,
	* testsuite/binutils-all/symbols-5.s: New test.